The metal oxide semiconductor (MOS) transistor, including N-channel MOS (NMOS), P-channel MOS (PMOS) and complementary MOS (CMOS), is one of the most important devices in integrated circuit technology today. And, because of the high loss of energy and the high generation of heat, the stability and the reliability of NMOS and PMOS are poor. They are gradually replaced by CMOS when the integration of the semiconductor device becomes higher and higher.
There are three types of CMOS devices: P-well type, N-well type and twin-wells type. The CMOS devices of P-well type are fabricated by forming N-channel devices in P-wells, while the P-channel devices are formed in the N-type substrate outside the P-wells. Contrarily, the CMOS devices of N-well type are fabricated by forming P-channel devices in N-wells, while the N-channel devices are formed in the P-type substrate outside the N-wells. The devices with twin-wells structure are fabricated with both types of wells built in either type substrate independently, while there is one MOS devices of opposite type formed in each well respectively.
The selection of the well types described above depends mostly on the circuit application. For example, the N-well technique is often used for fabricating dynamic random access memories (DRAMs) because P-channel devices have low substrate current whereas the high substrate current can be easily sunk from the P-type substrate. And the P-well technique has a benefit in its simpler fabrication because the P-well devices are less sensitive to field inversion and then the P-well itself can be used to be the channel stop for N-channel. The twin-wells structure, wherein the doping profiles in each well could be set independently to optimize both device types, is suitable to be used in the high integration circuit of sub-micron devices. In addition, the twin-wells structure has lower junction capacitance and lower body effect, and it has the flexibility of selecting substrate type without affecting performance. These advantages make the twin-wells a better structure than the N-well and the P-well structure.
Recently, there are three methods provided for fabricating the twin-wells structure These are disclosed in the following references Ruojia Lee, et al. in U. S. Pat. No. 5,024,961 titled "Blanket Punchthrough and Field-isolation Implant for Sub-micron N-channel CMOS Devices", Douglas C. H. Yu, et al. in "Low Threshold Voltage CMOS Devices with Smooth Topography for 1 Volt Application" IEDM Tech. Dig., Vol. 94, p. 489, (1994); and C. T. Liu, et al. in "0.2-.mu.m n-Channel and p-Channel MOSFET's Integrated on Oxidation-Planarized Twin-Tubs" IEEE Electron Device Letter, Vol. 17, p. 500 (1996). But all of these three methods will also have poor topographies, that is, a nonplanar surface occurs at or near the interface of the P and the N-well due to the high temperature oxidation used over one well. Although a height difference is useful for subsequent lithography for fabricating these devices, a large difference makes the transitional area between the P-well and N-well wasteful. Therefore, there is a need for a simplified and an improved method for fabricating CMOS devices with twin-wells structure.